Vitreous-bonded cubic boron nitride abrasive grinding system

ABSTRACT

A vitreous-bonded cubic boron nitride abrasive grinding system for wet grinding is described utilizing borosilicate glass in combination with a metal filler. The metal is selected from the class consisting of molybdenum, tungsten and alloys thereof.

United States Patent 119 Hibbs, Jr. et al. Dec. 3, 1974 s41-VITREOUS-BONDED CUBIC BORON 3,645,706 2/1972 Bovenkerk 51/298 NITRIDEABRASIVE GRINDING SY 3,650,714 3/1972 Fark as 3,664,819 5/1972 Sioun etal..

[75] Inventors: Louis E. Hibbs, Jr., Schenectady; 3,779,727 12/1973Siqui et a1. 51/298 Kenneth A. Darrow, Sprakers;

William R. Reed, Jr., Schenectady, 1

all of Primary Examiner-'Donald J. Arnold I Attorney, Agent, or FirmLeol. MaLossi; Joseph T.

[73] Asslgnee. General Electric Company, Cohen; Jerome C- SqumaroSchenectady, NY. [22] Filedz Apr. 2, 1973 [21] Appl. No .:'346, 920 [57]ABSTRACT [52] US. Cl. 51/308, 51/309 A vitreous-bonded cubic boronnitride abrasive grind- [51] Int. Cl C04b 31/16 ing system for wetgrinding is described utilizing boro- Field of Seam]! 8, 309, 98silicate glass in combination with a metal filler. The

Y I metal is selected from the classvconsisting of molybde- [56]References Cited num, tungsten and alloys thereof.

' UNITED STATES PATENTS 3,576,610 4/1971 Mathewson 51/308 3 Claims, NoDrawings VITREOUS-BONDED CUBIC BORON NITRIDE ABRASIVE GRINDING SYSTEMBACKGROUND OE THE INVENTION Individual cubic boron nitride grains coatedwith a thin layer of borosilicate are described in U.S. Pat. No.3,576,610 Mathewson.

The art is consistently in need of improved grinding capability formachining aerospace alloys. This invention relates to an improvedgrinding,system directed to this end.

SUMMARY OF THE INVENTION The instant invention is directed to avitreous-bonded cubic boron nitride abrasive grinding system for wetgrinding embodying borosilicate glass in combination with a metalfiller. The metal is selected from the class consisting of molybdenum,tungsten and alloys thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT On the premise that fillermaterials could be found that would have a beneficial effect on the CBNgrit/- borosilicate glass matrix bond, investigations were conducted toisolate any such fillers. It was anticipated that lead oxideborosilicate glass being used might not be stable when fired in anon-oxidizing atmosphere and, for that reason, the first materialsinvestigated were refractory filler materials;

The first filler material employed was alumina and the composition ofthis wheel (No. l is shown in Table IV below. The results of grindingwith wheel No. l are shown in Table V. The performance was so poor thatgrinding ratios .could not be obtained and microscopic examinationrevealed massive transfer of workpiece material to the wheel. Theindication was that a chemical reaction had occurred between the metalof the workpiece and either the glass or the alumina filler.

Next, wheels were constructed using only the borosilicate glass and CBNgrains; these wheels are indicated as Nos. 2 and 3 in Tables IV and V.When wheel No.

2 was fired at 950C, the CBN became thoroughly wetted by the glass,however, therim slumped when fired at 950C. Thereafter, the contents ofwheel No.2 was carefully crushed into a coarse powderincludingagglomerates of several of the original CBN crystals.Polymethylmethacrylate was added as a binder and the material wasremolded as wheel No. 3. The new binder enabled firing at a lowertemperature (775C 1/2 hour), the bulk of the CBN grit having alreadybeen glass-coated. Testing of wheel No. 3 (Table V) on Inconel 718clearly indicated'that the alumina filler (not the glass) used in wheelNo. 1 had been the major cause of loading of the wheel with metal fromthe workpiece.

, The experiments with wheels Nos. 1 and 3 established that a suitablefiller was, indeed, required, but the filler would have to verysignificantly reduce wheel loading. Because of the imposed restrictionof firing in an oxidizing atmosphere to preserve the properties of theborosilicate glass, a number of refractory compounds were. given apreliminary evaluation as shown in Table I. Each cylindrical pill samplewas pressed cold (pressure of about 5000 psi) and then transferred tothe furnace for firing. The cylindrical pill specimens were examined fordimensional changes occurring during firing and evidence of chemicalreactions (such as gas holes, porosity, friability, etc.). Each samplewas broken and the fractured surfaces examined to note any differencesbetween the interior and the exterior surfaces.

TABLE I CYLINDRICAL PILL SPECIMENS FIRED IN AIR (All specimens 67 v/ofiller. 33 v/o SGJ" glass. fired one-half hour at 950C) Filler MaterialRemarks W 8 some evidence of gas evolution; surface bubbles; very slightvolume increase;

gas evolution; formation of additional phase as white crystals onsurface; questionable usefulness.

* SG-7 glass lead oxide horosilicate glass manufactured byOwen-Illinois. Toledo. Ohio.

As may be seen from Table I the only refractory compounds that yieldedreasonably dense bodies were zirconium diboride and tungstenpentaboride. Thereafter, wheel No. 4 was made using zirconium diborideas the filler. All experimental grinding wheels were pressed hot (160C)using polymethylmethacrylate as the binder for green strength. Exceptwhere otherwise noted, the pressure employed was 5000 psi, the pressurebeing maintained until the mold had cooled below C. In each instance anabrasive rim was prepared (1 inch O.D. inch l.D. X )4; inch wide) andmounted on an aluminum alloy hub, using epoxy resin cement. All grindingtests were run under carefully controlled standardized conditions.

1. Standard Surface Grinding Test Conditions Wheel Speed 5,500 surfacefeet/minute Table Traverse Speed 18 feet/minute Crossfeed 0.050inch/pass Downfeed Variable (see Table V for values used) Grinding Wheel1 inch dia., A inch wide (nominal) Lubrication Mist, applied with highpressure air aspirator (-100 lbs.)

2-. Lubricants and Coolants Rust Lick A chemical grinding concentrate,di-

luted to 1 percent with water;

Vantrol 5495G A heavy duty sulfurized and chlorinated cutting oil,manufactured by Van Straaten Chemical Company, Chicago, Ill.

Sun Grind 600X A grinding oil specifically formulated for severeoperations such as thread grinding; less viscous than Vantrol 5495G;manufactured by Sun Oil Company; Philadelphia, Pa.

No. 4204 A napthenic mineral oil containing 4% iodine and bromine.

3. Workpiece Materials M2 High Speed Tool Steel (R, 60-61) lnconel 718(R,4244) Ni base high temperature,

high strength alloy; solution treated and aged.

A two-step process was used to true and open the surface of eachgrinding wheel before it was first tested. Both steps were, performeddry.

Truing was accomplished by running the wheel against a resin-bonded barcontaiging -40O mesh diamond abrasive. This bar was clamped in a fixturemounted on the grinder table. The wheel speed was 18,000 rpm; the tablespeed was 18 fpm; and the downfeed was 0.0001 inch. The crossfeed was0.050 inch per pass continued until the wheel was true. The crossfeedwas then reduced to 0.010 inch and an additional 0.0001 inch was removedfrom the wheel.

After the truing operation, the wheel surface was too smooth forgrinding. Therefore, the diamond tool was replaced with a Norton No. 37C220 kv silicon carbide dressing stick, and the previous operation wasrepeated. The wheel speed and table speed were the same as before andthe crossfeed was kept at 0.050'inch. The downfeed, which was started ata low value, was increased every few passes to a final value of 0.003inch. These conditions were maintained until 0.200 inch had been groundfrom the .silicon carbide stick, at which point the wheel bond had beeneroded back sufficiently to expose the abrasive grit and permit grindingon metal workpieces.

Additional grinding on tool steel at low downfeeds was usually necessaryto open the wheel surface sufficiently to make a grinding ratio test.

The testing of wheel No. 4 was disappointing, because although there wasno indication of transfer of metal to the wheel, the wheel was so softthat loading would not have been expected. Reexamination of the tungstenboride and zirconium boride pill specimens (Table I) showed a differencein appearance between the centers and the regions near and at thesurface. It was then decided that oxidation during firing was causingundesirable changes in the filler and tests were undertaken to see whataffect the use of a non-oxidizing atmosphere would have on theproperties of the glass matrix.

Table II shows the results of a re-evaluation of the refractorycompounds shown in Table I with the firing TABLE II ContinuedCYLINDRICAL PILL SPECIMENS FIRED IN NITROGEN (A HOUR AT 950C) apparentgreat evolution of gaseous products; very porous product; not usable.

Again, only the zirconium diboride and tungsten pentaboride samplesshowed promise. As the glass content was increased, the toughnessincreased and the degree of porosity decreased. Fortunately, it wasfound that the glass properties were not significantly affected by thechange in the firing regimen.

Wheels containing tungsten pentaboride (No. 8) and zirconium diboride(No. 9) as the filler materials were made and fired in a nitrogenfurnace atmosphere. The nominal composition of these wheels was 25 v/oCBN, l5 v/o filler, 30 v/o lead oxide borosilicate glass and 30 v/ovoids. The final compositions calculated from the final dimensions ofthe rims are given in Table IV. Although the results of grinding tests(Table V) showed that these wheels were superior to those obtained withvitreous bonds fired in air, the performance was still not good enough.

Without the previously imposed constraint of firing in an oxidizingatmosphere, it now became feasible to investigate the use of metal as afiller for the vitreousbonded CBN system. This change did offer thepossibility of a system in which the thermal expansion characteristicsof all the constituents could be essentially identical. The idea ofusing a metal filler did, however, run contrary to the prospect offurther reducing workpiecewheel interaction and the consequent metaltransfer.

Molybdenum (325 mesh, U.S. Sieve) was chosen for the preparation of aseries of pill specimens for preliminary evaluation as set forth inTable III. In each test the CBN grains were 120/140 mesh size (U.S.Sieve). It was determined that the useful glass-to-molybdenum ratioscould vary from about 2:1 to 1:2.

Examination of a polished section of sample 3 (Table III) under highmagnification (1,000X) shows the development of a thin layer at theglass/CBN interface, which is apparently different from either the glassor the CBN. This layer, the composition of which is unknown at present,may account for the tenacity of the bond. Lead oxide borosilicate glassemployed in sample 4 of Table III is a crystallizing glass with the samethermal expansion as the SG7 glass.

TABLE III CYLINDRICAL PILL SPECIMENS CONTAINING MOLYBDENUM,

METAL FILLER PLUS CBN I Porosity (v/o. calc.) Remarks now beingconducted in a non-oxidizing atmosphere.

TABLE II CYLINDRICAL PILL SPECIMENS FIRED IN NITROGEN (V4 HOUR AT 950C)Filler Volume Percent Material Filler Remarks ZrB 67 no significantvolume change; harder than equivalent sample fired in air;

ZrB, 5O slight volume decrease;

calculated density 70% of theoretical;

ZrB 33 slight volume decrease;

fractured specimen was hard. 5 tough; density 87% of theoretical value;

W 8 33 slight decrease in volume;

quite hard; no evidence of significant chemical reactions;

Sample Composition No. (volume percent) l 33 SG7 glass 67 Mo.

any specimens made from refractory fillers;

3,852,049 W M L... TABLE III-Continued CYLINDRICAL PILL SPECIMENSCONTAINING MOLYBDENUM. METAL FILLER PLUS CBN Sample Composition PorosityNo. (volume percent) (vlo, calc.) Remarks 2 37.5 SG-7 30 good wetting ofCBN by glass; fractured 37.5 Mo surface showed cleaved and broken grit,25.0 CBN indicating improved bond;

3 50.0 88-7 glass 30 wetting of CBN by glass improved over 25.0 Moprevious specimen; surface was tough, 25.0 CBN not friable;

4 50.0 CV 635 glass 30 no significant volume change on firing; 25.0 Moglass appeared to wet CBN; specimen was 25.0 CBN tough; broken andcleaved abrasive visible at fractured surface.

' The CV 635 (Owens-Illinois) sample was fired by'healirlg to 950C.holding for 5 minutes. cooling to 700C. holding for 1 hour. then coolingto room temperature.

Th results f th t st on th ill Specimens (T bl grinding test results areset forth in Tables IV and V be- III) were so promising that it was thde id d t k low. The nominal concentration of CBN abrasive in allgrinding wheels containing molybdenum as the filler. Wheels except Nos.2 and 3 was v/o. The composi- The compositions of the test wheels andcomparable tions given are those calculated from the final dimenvsionsof the completed rims.

TABLE IV Furnace \Vh l v/o Max. Firing Atmos- Nti Abrasive v v/o Fillerv/o Glass v/o porosny Ternp.C phere Remarks 1 CBN l7 Alundum 30.6 S C-713.6 39.4\ 950 Air -7 20 950 Air Rim sagged 2 CBN 4o so during firing. 3CBN 40 80-7 20 40 775 Air 4 CBN l6.8 ZrB, 31.9 56-7 l5.l 36.2 950 Air 5CBN i 27.7 Mo 16.7 507 33.3 22.3 950' N 6 CBN 25.0 Mo l5.0 80-7 30.030.0 950 N,

7 7 32.7 23.6' 950 N Rim accidentally I 7 B 5G 7 2 broken duringmounting on hub; not tested. 8 CBN 27.5 W43, 16.5 80-7 33.0 23.0 950 N,

9 CBN 27.4 Z rB, 16.4 56-7 32.9 23.3 950 N,

TABLE V Material Removal Rate 3 Downfeed Grinding Wheel No. workpiece(inlmin) (in.) Lubricant Ratio Remarks 1 M2 0.0081 0.0075 Rust Lickchatter, severe metal pickup; I In. 7 l 8 0.0054 0.0005 Vantrol chatter,metal pickup;

3 In. 718 0.0054 0.0005 Vantrol 4 performed well for min., then burst;

possible machine malfunction;

4 M2 0.0081 0.0075 Rust Lick 0.4 very soft wheel; no

4 evidence of loading;

9 M2 0.008] 0.00075 Vantrol 409 slight chatter;

' slight metal pickup;

8 M2 0.0054 0.0005 Vantrol 44 soft wheel; no

chatter;

TABLE V Continued Material Removal Rate 3 Downfeed Grinding Wheel No.workpiece (in /min) (in.) Lubricant Ratio Remarks M2 0.0054 0.0005Vantrol 1514 no obvious chatter or.

wheel loading; M2 0.081 0.00075 Vantrol 1520 M2 0.0108 0.001 Vantrol1990 1n. 718 0.0081 0.00075 Vantrol 5H 6 M2 0.0108 0.001 Vantrol 1940 noobvious chatter, no

metal pickup; In. 718 0.0081 0.00075 Vantrol 1394 1n. 718 0.0084 0.00075Sun Grind 173 wheel chattered; In. 718 0.0108 0.00] Vantrol 1192 nochatter or wheel loading.

Wheel No. 5 was tested first on M2 tool steel. There is no obviousexplanation for the fact that the performance at 0.00l-inch infeed wasbetter than the performance at smaller feeds.

Wheel No. 6 performed even better than No. 5. Performance at 0.00l-inchinfeed was better than the performance of wheel No. 5 at 0.00075-inchinfeed. The additional porosity, providing for better chip clearance andfor more penetration of the lubricant into the cut- 2 ting zone mayaccount for the improved grinding ratios.

the CBN crystals was evident. At higher magnification (lOOOX)extensively fractured abrasive particles were observed, which particleswere still firmly bonded to the matrix and capable of doing moregrinding work. There was no evidence of dull cutting edges and noobvious adhesion of metal from the workpiece to the abrasive. At veryhigh magnification (20,000X) a portion of the fractured surface of a CBNcrystal was examined and it was observed that the cleavage of CBN occursalong many well-defined planes permitting the abrasive grain to remainsharp and to grind efficiently as long as it is properly anchored in thewheel.

The role of the metal filler material in this vitreous system is notclearly understood. It is obvious, however, that the presence of themetal filler does serve to toughen and strengthen the borosilicatesglass matrix. It has been demonstrated that reaction between the fillerand workpiece causes excessive wheel loading and that a less reactivefiller (zirconium boridevis-a-vis aluminum oxide) would significantlyreduce metal transfer and improve grinding efficiency. According to theobvious reasoning the use of a metal filler, such as molybdenum ortungsten, would be expected to produce significant galling and welding,because of the greater reactivity of metals.

The unexpected behavior of the molybdenum filler can be hypothesizedafter the fact in that all the constituents making up the wheel areessentially matched with respect to thermal expansion (linearcoefficients of thermal expansion of 35, 45 and 40 X 10 in. per in. perdegree Centigrade for CBN, lead borosilicate glass, and molybdenum,respectively). The bond system is tougher, pullout of abrasive grains isreduced and the protrusion of the grit from the matrix is maintained sothat less frictional heat is generated by the rubbing of non-abrasiveconstituents on the workpiece. In addition, the metal filler impartsmuch greater thermal conductivity to the wheel, which serves to furtherreduce the temperature at the wheel workpiece interface. As a result,the interface temperature may be reduced to the extent that molybdenum,which is a refractory metal, does not react significantly with the metalworkpiece, if adequately lubricated.

Criteria for the selection of a borosilicate glass for use as the matrixin this system are:

l. The coefficient of thermal expansion should be almost identical tocubic boron nitride.

2. The glass, when molten, should exhibit a strong fluxing action, whichcleans contaminants from the abrasive particle surface, permittingwetting to take place.

3. The glass should have sufficient thermal stability to permit firing,for short periods of time, at temperatures up to 950C, much above therecommended temperature. At the higher temperatures the glass viscosityis greatly reduced, allowing the glass to penetrate minute crevices andcracks in the grit surface, resulting in a stronger bond.

4. The glass must tolerate firing in a neutral atmosphere, such asnitrogen, without significant impairment of properties.

The selection of the metal filler should be made in accordance with thefollowing criteria:

1. The coefficient of thermal expansion should be close to that of theglass and the abrasive, in order to minimize deleterious stresses.

2. The filler should be chemically compatible with the glass over theentire temperature range used for firing the composite structure.

3. The filler must be wet by the glass.

4. The filler should reduce the brittleness of the glass, limit thepropogation of cracks, and increase the impact strength andtoughness ofthe system.

5. The filler, in combination with the glass, should not exhibitsignificant tendency to react chemically with the workpiece under thegrinding conditions chosen, in order to minimize wheel pickup andloading.

6. The filler should improve the thermal conductivity of the grindingsystem to prevent overheating of the abrasive and the workpiece, thusreducing wheel wear and metallurgical damage to the material beingground.

Grinding wheels prepared according to the teachings of this inventionmay have compositions in which the constituents (as defined herein) andvoid content will be present in quantities (expressed as volume percent) in the ranges shown below:

Abrasive grit 6-307: Borosrhcate glass 20-50% Metal filler l0-2571 Voids10-3071 The amount of binder employed to provide green strengthdetennines the void content of the completed wheel. The binder materialshould be a depolymerizable polymer that is converted to monomerswithout leaving carbon residue. Carbon in contact with the borosilicateglass during firing will reduce the glass and destroy its properties.Polymethylmethacrylate and polyisobutylrnethacrylate are suitable bindermaterials. Other useful binder materials may be selected by reference toSection II (pages 389-95) of the Polymer Handbook [Edited by J. Brandrupet al, Interscience, 1966].

What we claim as new and desire to secure by Letters Patent of theUnited States is:

l. A vitreous-bonded abrasive article system comprising in combinationcubic boron nitride grains, a borosilicate glass and metal powderfiller, the metal being selected from the group consisting of molybdenumand alloys thereof.

2. The vitreous-bonded abrasive article recited in claim 1 wherein theabrasive system consists of the following materials (expressed in volumeper cent):

3. The vitreous-bonded abrasive article recited in claim 1 wherein themetal filler is molybdenum.

1. A VITREOUS-BONDED ABRASIVE ARTICLE SYSTEM COMPRISING IN COMBINATIONCUBIC BORON NITRIDE GRAINS, A BOROSILICATE GLASS AND METAL POWDERFILLER, THE METAL BEING SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUMAND ALLOYS THEREOF.
 2. The vitreous-bonded abrasive article recited inclaim 1 wherein the abrasive system consists of the following materials(expressed in volume per cent):
 3. The vitreous-bonded abrasive articlerecited in claim 1 wherein the metal filler is molybdenum.